The invention relates to a method of producing saw bands and saw blades, in particular bandsaw blades for planing, contouring, mitering and parting cuts.
Saw bands and saw blades must have high dimensional stability of their cutting edges as well as high wear resistance and be able to withstand the high loading produced by compressive, flexural and shearing forces even under the not inconsiderable temperatures which result from the friction between the saw blade and the material being sawn. This applies especially to bandsaw blades, which circulate at high speed and are subjected to great curvature where they are deflected.
Since it is difficult to combine the properties mentioned above in a single material, saw blade blanks nowadays usually comprise a relatively tough supporting band with high bending fatigue strength and a cutting-edge band of a high-speed or cold work steel that is less tough but highly wear resistant. The cutting-edge band is in this case of such a width that at least the teeth tips of the saw band or blade, or else the cutting teeth as a whole, can be cut out from it.
It is known to join the supporting band and the cutting-edge band to each other by laser or electron-beam welding without filler material. From the bi-metal blank produced in this way, the teeth are cut out on the cutting edge side after annealing to eliminate welding stresses. This involves bending the teeth away alternately to one side and to the other side, to a greater or lesser extent out of the plane of the band, by setting. The setting is followed by subsequent austenitizing with water quenching and tempering, repeatedly if need be, for adjusting the wear resistance of the teeth or teeth tips.
It has been found in practice that distortion of the teeth or teeth tips often occurs during heat treatment, so that they no longer assume the intended angular position with respect to the plane of the saw band or saw blade and are consequently subjected to considerably greater wear. The local differences in concentration in the region of the weld have been found to be a cause of this. Such differences in concentration and, in particular, the concentration gradient transversely to the weld inevitably created when welding two materials of different compositions often lead to residual stresses and consequently to impairment of the service life. Added to this is the fact that, in the subsequent heat treatment, with its sometimes abrupt temperature changes, in particular during quenching, excessive local internal stresses often occur in the region of the weld or in the transition from one material to the other, lead to cracking and likewise impair the service life as a result of the crack propagation caused by the high flexural forces.
To improve weldability, Austrian Patent 398 176 proposes the use of a cutting-tool steel alloy with 0.7 to 2.3% carbon, 0.08 to 2.0% aluminum and 0.7 to 6.5% vanadium as well as a supporting steel with 0.15 to 0.50% carbon, 0.015 to 0.18% aluminum and 0.10 to 0.6% vanadium. With the relatively high contents of carbon and aluminum and the simultaneous presence of vanadium in the cutting-tool steel alloy, it is intended to influence the transformation behavior in such a way that higher tempering temperatures, and accordingly a higher secondary hardness and wear resistance, are possible. The relatively high aluminum content is intended in this case to ensure a higher carbon content without the risk of graphite precipitation, and accordingly a higher carbide fraction with correspondingly higher wear resistance.
Furthermore, U.S. Pat. No. 3,034,379 discloses the proposal of arranging between the cutting-edge steel band and the supporting band a thin strip of a heat-resistant steel of high hardness and low thermal conductivity, for example of an austenitic steel with a least 12% chromium. This procedure is very complex, however, because it requires not only precise insertion of the intermediate strip but also two welds. What is more, the welding of three materials of different compositions leads to even greater differences in concentration, with the disadvantageous consequences discussed above in conjunction with producing bi-metal blanks.
The invention is therefore based on the problem of finding a way of eliminating the disadvantageous inhomogeneities in the region of the weld and in this a way extending the service life of the saw bands and blades.
This problem is solved by a method in which a supporting band of relatively tough steel, in particular of high bending fatigue strength, and a cutting-edge steel band or blade of a high-speed steel are welded to each other and the weld is thereby adjusted to a ratio of its average width to the thickness of the band
Bm/D=0.30 to 0.9.
Saw bands and blades produced by the method according to the invention are distinguished by high dimensional and setting stability, i.e. the saw teeth remain in their intended position in spite of high loading and high working temperatures; their wear is therefore comparatively low.
The supporting band steel preferably contains 0.2 to 0.6% carbon, 0.5 to 6%, preferably at most 1.5%, chromium as well as up to 3.5% tungsten, molybdenum and vanadium, individually or in addition to one another, preferably 1.5 to 2.5% molybdenum and 0.10 to 0.75% vanadium, even if carbon contents of at least 0.4% or else 0.45 to 0.55%, preferably 0.35%, have proven to be particularly successful. The total content of alloying agents is preferably 3.12 to 8.48%.
Particularly suited is a supporting band steel with 0.32% carbon, 0.35% silicon, 0.70% manganese, 1.00% chromium, 2.00% molybdenum, 0.6% nickel, 0.15% vanadium and 0.05% tungsten, the remainder being iron.
The invention is based on the realization that the geometry of the weld is of decisive significance with regard to the service life of the saw bands and blades.
So far, relatively narrow welds of the order of magnitude of 0.25 mm have been used in practice, with a view to as small a proportion of cutting-tool steel as possible, and the thickness of the band has not been given the necessary attention. This is so because small weld widths are inevitably accompanied by a correspondingly low heat input during welding and a similarly small amount of liquid phase of the two adjacent materials. Since the intermixing of the two materials of different kinds takes place essentially by means of diffusion and the diffusion is both time-dependent and temperature-dependent, in the customary welding methods there is generally not adequate intermixing in the liquid phase and accordingly also not a smooth or continuous change in concentration in the region of the transition from one material to the other. This disadvantage is all the more serious the thicker the two bands to be welded to each other; this is so since the relatively large mass of the cold materials neighboring the weld on both sides, said mass increasing with the thickness of the band, exerts a strong quenching effect on the welds and leads to very steep temperature and viscosity gradients. This has the consequence of the inhomogeneities mentioned above in the region of the weld and the accompanying defects that greatly impair the service life.
By contrast, the invention teaches a geometry of the weld which takes the thickness of the band into account and ensures that an amount of liquid melt that ensures adequate concentration equalization is produced during welding. This applies especially to the carbide-forming elements, and the resultant carbides, ensuring high wear resistance.
Laser welding is especially suitable for the method according to the invention, because it works with inert gas, generally argon or helium, and therefore not only prevents oxidation of the melt but also promotes degassing of the latter, which is of particular advantage in the case of highly alloyed high-speed steels and powder-metallurgically produced steels in particular. Also suited, however, are other welding methods, in particular electron-beam welding, if the electron beam is in this case defocused in such a way that the width required for the geometry of the weld according to the invention is obtained.
The following high-speed steels come into consideration as the cutting-tool steel:
The cutting-tool steel contains up to a total of 20% tungsten, molybdenum, vanadium and niobium; however, it may also have greater contents of these alloying constituents within the scope of the invention. The minimum content should, however, lie above 7%.
The cutting-tool steel preferably contains a total of at least 8% or else at least 12% tungsten, molybdenum, vanadium and niobium.
The method according to the invention is also suitable in particular for use of particularly wear-resistant powder-metallurgically produced high-speed steels.
The saw band or blade blank is preferably subjected to stress-relief annealing before the shaping operation. After setting, the saw band or blade is normally austenitized, quenched and then temperedxe2x80x94repeatedly if need be.